1. Field of Invention
The present invention is related to digital to analog converter (DAC) input data encryption and decryption in which leakage of input data in-band harmonics is suppressed through input data encryption off-chip. More specifically, the present invention relates to the method and apparatus of input data encryption off-chip by forming the logical exclusive-OR of the raw data and a random single bit data stream. The encrypted data is then transferred onto the DAC chip where decryption occurs through the use of identical circuitry.
2. Description of Related Art
As an increasingly versatile device, digital to analog converters (DAC) are being found in a variety of applications and technologies. For example, many audio systems depend on exacting DAC performance to translate the binary words of tapes or discs into analog signals accurately reflecting the stored data. As the need for converters has increased, improvements to DAC technology have also increased. For instance, audio DAC technology has evolved from 14-bit converters to 16, 18 and even 20-bit converters, fabricated into flat-packs, dual-in-line packs or other convenient packages, made of plastic or ceramic, with isolated and non-isolated pins and a variety of other features.
Through similar measures, improvements to DAC performance have also been demanded, such as improved fan-out and propagation delay. Performance improvements have also included resolving many of the problems associated with smaller and smaller IC packages. For instance, the movement of data within IC packages has typically created several problems, such as crosstalk and transmission line reflections. Another problem associated with data movement within IC packages is leakage surrounding activated digit and word transmission lines. Binary data, consisting of a sufficiently high voltage, will create leakage into surrounding fields unless prevention measures are taken.
As pointed out in U.S. Pat. No. 5,245,569 issued Sep. 14, 1993 to Gonzalez et al., a traditional solution to prevent leakage from data and word lines within IC packages has been the use of long, thick field insulating oxide layers around data and word lines. However, as IC packages have grown smaller and smaller, the use of sufficiently thick field insulating oxide layers becomes impossible. Therefore Gonzalez et al. teaches a method of protecting digit and word lines from one another in IC packages through the use of an isolation voltage applied to surrounding inactive digit and word lines. Digit and word lines not in use, but immediately adjacent to lines in use, are charged with an isolating voltage which prevents leakage from the lines in use to the surrounding fields.
Another problem associated with data movement within IC packages is the detrimental effects certain digital signal frequency components may have on analog signals, primarily within mixed-signal analog-to-digital converters. As pointed out in U.S. Pat. No. 5,793,318 issued Aug. 11, 1998 to Robert E. Jewett, standard solutions such as shielding taught by Gonzalez et al., are often insufficient due to size restrictions or operating frequencies. Therefore Jewett teaches a method of eliminating crosstalk by encoding the output signal of an analog-to-digital converter and removing all correlation between the analog input signal and the encoded digital output signal. Jewett defines crosstalk as undesired noise appearing in one signal path, such as the digital output, resulting from the coupling of one signal path to another, such as the analog input. Encoding the digital output removes all correlation between coupled input and output signals, eliminating crosstalk.
The encoding consists of an exclusive-OR examination of a single bit raw digital output signal of the ADC and a pseudo-random number to encode the single bit digital output signal, preventing any coherence between the encoded digital output signal and the analog input signal. The exclusive-OR encryption eliminates crosstalk by removing all correlation associated with input/output coupling in analog-to-digital conversions. However Jewett and Gonzalez et al. fail to address the same problems in digital-to-analog conversions. Therefore what is needed is a method and apparatus to suppress the package related leakage of the in-band harmonics of n-bit data in digital-to-analog converters.